1. Field of the Invention
The present invention relates to a hydraulic brake control system for a vehicle with, such as, an anti-lock or anti-skid brake system and/or a traction control system.
2. Description of the Prior Art
U.S. Pat. No. 5,031,969, and its corresponding Japanese Unexamined Patent Publication No. 2-234868, disclose a device for controlling hydraulic pressure in a hydraulic system, such as, a vehicular hydraulic brake system with a skid control system. The disclosed device includes, as the characteristic structure, a throttle switching valve interposed between a buffer chamber and a brake master cylinder, and a buffer means provided in the buffer chamber is formed by a preloaded spring-biased device.
In another system, Japanese Unexamined Utility Model Publication No. 2-54769 discloses an anti-skid brake system, wherein an accumulator is provided for accumulating brake fluid. The brake fluid is first pumped from a reservoir tank to the accumulator and then returned to a first hydraulic line extending between a brake master cylinder and a brake wheel cylinder. The disclosed system further includes a variable throttle in the first hydraulic line which changes its throttling degree depending on the displacement of the piston of the accumulator.
The foregoing conventional techniques are aimed respectively at eliminating pulsation transmitted to a brake pedal due to hydraulic pressure fluctuation caused by. However, large noises generated by the pump being driven at a relatively high speed still remains a problem.
Specifically, in the conventional systems, the reciprocating pump piston operates at about 60 Hz. Consequently, large noises are generated at integral multiples of the fundamental frequency 60 Hz so as to vibrate the air in audible frequency band.
Accordingly, to suppress effectively the noise induced by the operation of the pump, it is advantageous to prolong the period of the reciprocating piston. In other words reducing the frequency of the reciprocating piston by decreasing the speed of the pump, reduces the pump-induced vibration to less than the audible frequency band.
However, in conventional systems, the pump speed can not be decreased, i.e. the period of the reciprocating piston can not be increased due to the following reasons:
An oil consumption quantity or hydraulic consumption Q.sub.S (cm.sup.3) and a theoretical discharge rate q (cm.sup.3 /sec) of the pump in an anti-lock brake system are preset respectively according to a vehicle specification. The hydraulic consumption Q.sub.S represents a sum of the reservoir capacity Q.sub.R (cm.sup.3) and a pump chamber capacity (cm.sup.3). Accordingly, if the period of the reciprocating piston is prolonged, a hydraulic discharge quantity for each piston discharge stroke should be increased to satisfy the preset theoretical discharge rate q. This increase in each piston discharge quantity increases the hydraulic quantity flowing into the pump chamber such that the hydraulic consumption Q.sub.S becomes greater than the preset value. This consequence is significant when the hydraulic fluid flowing into the pump chamber supplies the chamber's maximum capacity. On the other hand, if the reservoir capacity Q.sub.R is decreased to prevent the hydraulic consumption Q.sub.S from becoming greater than the preset value, the hydraulic consumption Q.sub.S becomes smaller than the preset value. This consequence is significant when the hydraulic fluid flowing into the pump chamber supplies only the chamber's minimum capacity.
When the hydraulic consumption Q.sub.S becomes greater than the preset value, the following problem occurs:
When a failure occurs, for example, at a pressure-reduction valve to thereby establish communication between the master cylinder and the pressure-reduction system, braking becomes inoperative when the brake pedal bottoms to disable the pressure buildup in the master cylinder.
On the other hand, when the hydraulic consumption Q.sub.S becomes smaller than the preset value, the following problem occurs:
When a wheel is locked during braking a pressure-reduction valve is opened to remove the brake fluid in a corresponding wheel cylinder thereby releasing the locked wheel. Yet, when the hydraulic consumption Q.sub.S is smaller than the preset value, the brake fluid to be removed becomes smaller so that the pressure-reduction performance needed to release the locked wheel is lowered.